Draw up catalog.json format version 3

[piroor]

Known problems in v2 (should be fixed at version 3):

• "replicas" appears in a dataset definition directly. It should be placed under "volume".

• We cannot put replicas under a slice. (note: actually this can be done on the latest implementation for v2, but it is invalid as a v2 catalog.)

• "volume" should be optional. If a slice doesn't have "volume", the slice itself should be parsed as a volume.
  Otherwise, we'll see deeply nested JSONs like: "dataset-volume-replicas-slices-slice-volume-replicas-..."

• Replicas according to application by use. For example:

  • Replica 1: There is only one slice. This should be used for generic search.
  • Replica 2: There are time-sliced slices. This should be used for search with year.
  • Replica 3: There are category-sliced slices. This should be used for search with category.

  For these replicas, search requests should be delivered only for suitable replica for the use case.